Skull spider powers are listed on the BIONICLE web page. The green ones have x-ray vision, the silver ones are fast, and the blue ones are tough (hard to kill).

What happens when they attach to someone's face, though, is still (as far as I know) not known. But it's probably reasonable to assume that they at least take control of the host.

The gearbox attachment is compatible with pretty much any HF torso beam (from the smallest torso beam used for the Protectors to the largest used in sets like Fire Lord or Black Phantom). And you can conceivably make use of it even without a standard torso beam as well. You'd probably have less luck using it with old Bionicle torsos, but that's more a fault of those torsos and their complex, hard-to-expand shapes than of the gearbox itself.

Frankly, I wouldn't be surprised if it even made its way back into some smaller Technic set.